[
https://issues.apache.org/jira/browse/HDFS-6010?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13932891#comment-13932891
]
Yu Li commented on HDFS-6010:
-----------------------------
{quote}
You know how things work when there are deadlines to meet
{quote}
Totally understand, no problem :-)
----
{quote}
1. How would you maintain the mapping of files to groups?
{quote}
We don't maintain the mapping in HDFS, but use the regionserver group
information. Or say, in our use case, this is used along with the regionserver
group feature, the admin can get the RS group information through a hbase shell
command, and pass the server list to balancer. To make it easier, we actually
wrote a simple script to do the whole process, while admin only need to enter a
RS group name for data balancing. More details please refer to answer of
question #4
\\
{quote}
wondering whether it makes sense to have the tool take paths for balancing as
opposed to servers
{quote}
In our hbase use case, this is Ok. But I think it might be better to make the
tool more general. There might be other scenarios requring balancing data among
subset instead of fullset of datanodes, although I cannot give one for now. :-)
----
{quote}
2. Are these mappings set up by some admin?
{quote}
Yes according to above comments
----
{quote}
3. Would you expand a group when it is nearing capacity?
{quote}
Yes, we could change the setting of one RS group, like moving one RS from
groupA to groupB, then we would need to use the HDFS-6012 tool to move blocks
to assure "group-block-locality". We'll come back more about this topic in
answer of question #5
----
{quote}
4. How does someone like HBase use this? Is HBase going to have visibility into
the mappings as well (to take care of HBASE-6721 and favored-nodes for writes)?
{quote}
Yes, through HBASE-6721(actually we have done quite some improvements to it to
make it simplier and more suitable to use in our product env, but that's
another topic and won't discuss here:-)) we could group RS to supply
multi-tenant service, one application would use one RS group(regions of all
tables of this application would be served only by RS in its own group), and
would write data to the mapping DN through favored-node feature. To be more
specific, it's an "app-regionserverGroup-datanodeGroup" mapping, all hfiles of
the table of one application would locate only on the DNs of the RS group.
----
{quote}
5. Would you need a higher level balancer for keeping the whole cluster
balanced (do migrations of blocks associated with certain paths from one group
to another)? Otherwise, there would be skews in the block distribution.
{quote}
You really have got the point here:-) Actually the most downside of this
solution for io isolation is that it will cause data imbalance in the view of
the whole HDFS cluster. In our use case, we recommend admin not to use balancer
over all DNs. Instead, like mentioned in answer of question #3, if we find some
group with high disk usage while another group relatively "empty", admin can
reset the group to move one RS/DN server around. HDFS-6010 tool plus HDFS-6012
tool would make the trick work.
----
{quote}
6. When there is a failure of a datanode in a group, how would you choose which
datanodes to replicate the blocks to. The choice would be somewhat important
given that some target datanodes might be busy serving requests
{quote}
Currently we don't control the replication of failed datanodes, but use the
HDFS default policy. So the only impact datanode failure does for isolation is
that the blocks might be replicated outside the group, that's why we need
HDFS-6012 tool to periodly check and move "cross-group" blocks back
----
[~devaraj] hope the above comments could answer your questions, and feel free
to let me know if any further comments. :-)
> Make balancer able to balance data among specified servers
> ----------------------------------------------------------
>
> Key: HDFS-6010
> URL: https://issues.apache.org/jira/browse/HDFS-6010
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: balancer
> Affects Versions: 2.3.0
> Reporter: Yu Li
> Assignee: Yu Li
> Priority: Minor
> Attachments: HDFS-6010-trunk.patch
>
>
> Currently, the balancer tool balances data among all datanodes. However, in
> some particular case, we would need to balance data only among specified
> nodes instead of the whole set.
> In this JIRA, a new "-servers" option would be introduced to implement this.
--
This message was sent by Atlassian JIRA
(v6.2#6252)
